1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying apparatus, a printer or a facsimile apparatus, and more particularly to an image forming apparatus adapted for transferring an image on an image bearing member onto an intermediate transfer member and then transferring the image on the intermediate transfer member onto a recording material.
2. Related Background Art
FIG. 7 shows, in a lateral views, an image forming apparatus for sequentially superposing toner images of plural colors on a same recording material.
In the main body of the image forming apparatus, there is provided an endless intermediate transfer belt 81 running in a direction indicated by the arrow X. Such intermediate transfer belt is formed with a film of dielectric resin such as polycarbonate, polyethylene terephthalate or polyvinylidene fluoride. A recording material 6 such as paper taken out from a feed cassette 60 is supplied, through registration rollers 13 and a conveyor belt 8, to a secondary transfer position of the intermediate transfer belt 81.
Above the intermediate transfer belt 81, there are provided four image forming units Pa, Pb, Pc and Pd in tandem. These image forming units Pa, Pb, Pc, Pd have substantially same structures are different in forming magenta, cyan, yellow and black toner images respectively.
The structure of the image forming unit will be explained in the following, taking the unit Pa as an example. The image forming unit Pa is provided with a rotatably supported drum-shaped electrophotographic photosensitive member (photosensitive drum) 1a, around which provided are process devices such as a primary charger 2a, a developing device 3a, a cleaner 5a etc. Since these units are similar in structure, the photosensitive drums alone are numbered as 1b, 1c, 1d in other units Pb, Pc, Pd. The developing devices of the image forming units Pa, Pb, Pc and Pd respectively contain magenta toner, cyan toner, yellow toner and black toner.
The photosensitive drum 1a is irradiated with a laser beam bearing an image signal of the magenta component of the original for example through a polygon mirror (not shown), whereby an electrostatic latent image is formed on the photosensitive drum 1a. The latent image is developed with magenta toner supplied from the developing device 3a to obtain a visible magenta toner image. When the toner image is brought, by the rotation of the photosensitive drum la, to a primary transfer position where the photosensitive drum 1a is in contact with the intermediate transfer belt 81, the magenta toner image on the photosensitive drum 1a is transferred onto the intermediate transfer belt 81 by a primary transfer bias applied to a transfer charging member 4a (primary transfer).
By the time a portion of the intermediate transfer belt 81 bearing the magenta toner image moves to the image forming unit Pb, a cyan toner image is formed on the photosensitive drum 1b in the image forming unit Pb in the same manner as explained in the foregoing and such cyan toner image is transferred onto the magenta toner image borne on the intermediate transfer belt 81.
As the intermediate transfer belt 81 moves further, a yellow toner image and a black toner image are similarly transferred in the respective primary transfer portions of the image forming units Pc, Pd onto the aforementioned magenta and cyan toner images. By this time, the recording material 6 supplied from the feed cassette 60 reaches a secondary transfer position through the registration rollers 13, and the four-color toner images on the intermediate transfer belt 81 are collectively transferred onto the recording material 6 by a secondary transfer bias applied to a secondary transfer member 40 (secondary transfer).
After the secondary transfer process, the intermediate transfer belt 81 is subjected to the surface charge elimination by a residual charge eliminator 12, and the secondary untransferred toner remaining on the surface is removed by a cleaning member 16.
The recording material 6 bearing the transferred four-color toner images is separated by a separating portion 10 from the intermediate transfer belt 81 and is conveyed by a conveying belt 8 to a fixing device 11, in which a pair of rollers apply heat and pressure to the recording material 6, thereby fixing the toner images. The fixing device 11 is provided a mechanism for coating releasing oil (such as silicone oil) on one of the fixing rollers in order to improve the releasing property between the recording material and the roller, so that such oil is deposited onto the recording material.
In case of one-side image formation, the recording material 6 bearing the fixed toner image is discharged onto a sheet discharge tray of the apparatus after emerging from the fixing device 11, but, in case of two-side image formation the recording material 6 is contained in a cassette 62 for two-side recording after passing through a surface reversing path 61 provided at the exit of the fixing device 11 and is then fed to the secondary transfer position thereafter for image formation by secondarily transferring the toner image on the other surface (back surface) of the recording material.
The photosensitive drum 1a after the primary transfer is subjected to the removal of the toner remaining on the surface thereof after the transfer by a cleaner 51, and is thus rendered capable of the next image formation. The intermediate transfer belt 81 after the secondary transfer is subjected to the removal of surface charge and of the toner deposited on the surface charge and of the toner deposited on the surface by a residual charge eliminating charger 12 disposed downstream of the secondary transfer position and a belt cleaner 16 disposed downstream of the residual charge eliminating charger 12.
In the above-described image forming apparatus, when a high voltage is repeatedly applied to the intermediate transfer belt in the respective primary and secondary transfer portions, the surface resistivity of the intermediate transfer belt may gradually decrease to eventually result in an image defect. This phenomenon is presumably attributable to a fact that discharge phenomena appear not negligibly by an electric field formed between the transfer rollers 4a, 4b, 4c, 4d and the photosensitive drums 1a-1d across the intermediate transfer belt (discharge being presumably generated in gap portions formed at the upstream and downstream sides of an area where the photosensitive drum is in contact with the intermediate transfer belt in each primary transfer portion), thereby deteriorating the surface layer (at the image bearing side) of the intermediate transfer belt or causing deposition of the discharge products onto the surface of the intermediate discharge belt. Such discharge phenomena are presumed to similarly occur also in the secondary transfer portions
In fact, in a durability test by current supply to the primary and secondary transfer chargers in the above-described image forming apparatus, the surface resistivity of the intermediate transfer belt was lowered from an initial value of about 1013 xcexa9/xe2x96xa1 to about 109 xcexa9/xe2x96xa1 with the progress of the durability test (measured under normal temperature and normal humidity conditions, with a voltage of 1 kV applied by an electrode according to JIS-K6911). With such loss of the surface resistivity, the volume resistivity was also lowered from about 1012 xcexa9cm to about 1010 xcexa9cm (measured under normal temperature and normal humidity conditions, with a voltage of 10 V applied by an electrode according to JIS-K6911). Furthermore, this phenomenon does not necessarily occur uniformly in the longitudinal direction (direction perpendicular to the rotating direction) of the intermediate transfer belt.
Such loss in the surface resistivity of the intermediate transfer belt may result in an image defect such as local blank area in the toner image transferred in each primary transfer portion from the photosensitive drum to the intermediate transfer belt, or a counter transfer of the toner, already transferred to the intermediate transfer belt, again to the photosensitive drum. Also in the secondary transfer portion, there may result an image defect such as a local blank area in the toner image transferred from the intermediate transfer belt to the recording material.
Also with the loss in the surface resistivity of the intermediate transfer belt in the prolonged use, the intermediate transfer belt becomes incapable of satisfactorily bearing the toner image electrostatically, leading to an image distortion called xe2x80x9csmeared imagexe2x80x9d.
Also in case of two-side image formation on the front and back surfaces of the recording material 6, there may be encountered the following drawbacks.
In the image fixation in the image formation on the front surface of the recording material 6, the releasing oil that has not penetrated in the recording material may remain on the surface thereof. If the image formation is executed on the back surface of the recording material in such state, the front surface of the recording material comes into contact with the surface of the conveying belt 8 so that, by the contact of the conveying belt 8 with the intermediate transfer belt 81 on the secondary transfer, the releasing oil is eventually transferred onto the surface of the intermediate transfer belt 81.
The releasing oil transferred onto the intermediate transfer belt 81 may be transferred onto the photosensitive drum 1 (1a-1d), by contact of the intermediate transfer belt 81 therewith in an area where the recording material 6 is absent in the course of image formation or in other operation, thereby resulting in a defective image caused by defective charging of the photosensitive drum, defective cleaning or defective development.
For example, if the image formation is executed on a small-sized sheet after two-side image formation on a large-sized sheet, the intermediate transfer belt bearing the releasing oil comes close to or contacts the photosensitive drum at the portion of the intermediate transfer belt out of contact with the recording material, whereby the releasing oil is transferred onto the photosensitive drum. In such situation, a next image formation on a large-sized sheet may show deterioration of the quality of the toner image formed on the photosensitive drum bearing thus transferred releasing oil.
An object of the present invention is to provide an image forming apparatus capable of preventing image defect caused by transfer of the oil, deposited on the intermediate transfer member, to the image bearing member.
Other objects of the present invention, and the features thereof, will become fully apparent from the following detailed description.